Tipping paper is employed to wrap cigarettes or other smoking products. Holes or perforations are formed in the tipping paper to allow cool atmospheric air to enter the tobacco smoke stream. Known methods of perforating tipping paper include pinned roller perforators and laser perforators. These systems may form lines of perforations in an elongated tipping paper strip or web, for example, at a frequency of 50 holes per inch.
The size and spacing of the holes or perforations in the tipping paper determine its permeability to the air and effect the degree of dilution of the smoke with atmospheric air. It is desirable to provide a constant permeability or dilution for particular types or brands of smoking products.
It is known to control laser perforators in response to permeability measurements made. See U.S. Pat. Nos. 4,569,359; 4,121,595, 4,648,412 and 5,092,350. Known permeability measuring devices include pneumatic systems for measuring the pressure drop through the tipping paper. However, such pneumatic systems are frequently inaccurate and difficult to implement in a high volume production line where the web can travel through the perforator at speeds of 5000 to 6000 ft. per minute.
Pneumatic measurements are frequently made off-line on a sample basis. In some prior art production lines, quality monitoring and control are accomplished through a combination of sampling and perforator adjustments. Initial setup can be accomplished by iterative trial and error in which the focus and power settings of the laser perforator are adjusted. After making tentative settings, the line is run to generate samples. The resulting samples are then tested in a pneumatic pressure drop instrument gage. Once the desired operating results are achieved, a manufacturing inspector periodically samples the perforated product, for example, a sample could be taken of five foot sections of paper from the end of every third bobbin to check for correct pressure drop. The paper could also be inspected by visual monitoring by holding the paper up to light to check generally for hole position and size. However, since such measurements are neither continuous nor in real time, defective perforation, if detected at all, would be determined after a large quantity of tipping paper has been perforated.
Optical monitoring devices for tipping paper perforation lines are also known. U.S. Pat. No. 4,569,359 includes a brief mention of optical monitoring. A known optical system for monitoring a perforation line is illustrated in FIG. 3 and described below. While such a system permits on-line monitoring of the process, in practice the output signal from this system has been found to correlate poorly with the pressure drops measured directly with pneumatic systems. Moreover, the system is affected by variations in the paper base sheet such as splices, extraneous holes, or thickness changes.